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UHF and VHF communications transmits generally by what we call “line of sight”. This is why they are only effective at short distances. Now HF (160m to 10m) on the other hand uses propagation. This is where the signal reaches the ionosphere and bounces back to earth, giving you greater distances, then what line of sight offers.
Ionosphere This is a layer of the atmosphere that lies anywhere from 80 miles to 300 miles above the earth’s surface. It’s responsible for reflecting radio waves around the world, and plays an important role in making DX calls. As the sun beats down upon the earth, it can change the composition and height of the ionosphere, which in turns changes the propagation characteristics. In general signals below 30MHz, will bounce of this part of the atmosphere, and travel back to earth. On the other hand, anything above 30MHz, will go right through and into space. So if your on 2m, you could talk to the ISS, or even maybe E.T himself. Also signals below 30MHz travel along the ground, and can be picked up short distances from the transceiver, as mentioned before. Daily Patterns Like I just mentioned, the sun can change propagation patterns. Also seasons can play a big role in making a DX contact. During the entire 24hr day, the ionosphere is changing. This will cause some signals to be bounced back to earth, while others are absorbed A general rule of thumb is that during daytime hours higher frequencies will propagate, and lower ones (below 10MHz) are absorbed into the atmosphere. At night this is reversed. Note though there are many exceptions to this depending on what frequencies you use, but this is an acceptable general guideline. Seasons Seasons can also play a big role. During winter months lower frequencies propagate better, and the reverse is true for summer. One of the best times for DXing is during season change, between fall and winter, and winter to spring. This is because seasonal changes are occurring in both the southern hemisphere and northern. Sunspots Every 11 years the sun goes through it’s sun spot cycle. In 2000 it was at it peaked, and come 2011 will be again, and then in 2022, 2033 and so on. this greatly affects propagation. A sunspot low occurs around midnight, during this cycle. When sunspots are at their maximum, propagation is best. Higher frequencies exhibit better propagation, even all the way down to 6m, which becomes highly active for Hams. Even 10m can be heard around the world QRPing. What’s the big deal? If you want to make that DX QSO, propagation is extremely important. You need to be aware of it, to make that contact. One way to do this is listen to the airwaves. Another is checking web sites that offer propagation information. You can even check it on this site, by clicking here. Other ways is through magazines, they offer charts that tell you the bands and best times to make that QSO. An excellent source is a DX Cluster, it tells you what’s going on right now.
UHF / VHF Normally not known for propagation, there are times when the signals travel further then expected. These times are far and few, and not as predictable as HF propagation. Some things that can cause this are........... -TROPO: This refers to the troposphere, which is mostly affected by weather patterns. Look for significant weather events, such as a cold front or major storms, Also early evenings and mornings are good time for this, as the earth is either cooling down or warming up at these times -SPORADIC E: this happens when small patches of the ionosphere is ionized. Normally this occurs in June, July, Early August, December, and January. As the name suggests, this is sporadic, so you never know when it may happen, so patience is of essence, when dealing with Sporadic E. -AURORA: The northern lights are caused by a solar storm and can have an effect on radio propagation. Not only can it affect VHF and UHF, it can also affect HF. To use the aurora you may need to point your antenna north to work a station to the south as signals bounce back from the aurora, vice versa if you live in the southern hemisphere. |
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